Calmodulin (CaM) double mutant T34C; T110C is randomly labeled at two positions with fluorescence donor and non-fluorescent acceptor molecules. We then measure the lifetime of the donor ( covalently linked to CaM) in the absence and presence of a peptide analog of the calmodulin binding domain from nitric oxide synthase. From lifetime changes we use FRET theory to calculate a distance between the donor and acceptor in the presence and absence of the peptide and, therefore; infer changes in the conformation of CaM.